sata_via: Delay on vt6420 when starting ATAPI DMA write

When writing a disc on certain lite-on dvd-writers (also rebadged
as optiarc/LG/...) connected to a vt6420, the ATAPI CDB ends
up in the datastream and on the disc, causing silent corruption.
Delaying between sending the CDB and starting DMA seems to
prevent this.

I do not know if there are burners that do not suffer from
this, but the patch should be safe for those as well.

There are many reports of this issue, but AFAICT no solution was
found before. For example:
http://lkml.indiana.edu/hypermail/linux/kernel/0802.3/0561.html

Signed-off-by: Bart Hartgers <bart.hartgers@gmail.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>